// 返回顶部
  var  returnBtn = document.getElementsByClassName('return_top')[0];
  var clientHeight = document.documentElement.clientHeight;
  var scrollTime;
  var isSro = true;
  window.onscroll = function()
  {
      if(document.documentElement.scrollTop <= clientHeight*1 )  returnBtn.style.display = 'none';
      else  returnBtn.style.display = 'block';

      if(!isSro) clearInterval(scrollTime);
      isSro = false ;
  }
  returnBtn.onclick = function()
  {
      scrollTime=setInterval(function() {
          var scrollDistance = document.documentElement.scrollTop;
          document.documentElement.scrollTop = scrollDistance + Math.floor(-scrollDistance/5) ;
          if(scrollDistance<=0) clearInterval(scrollTime);
          else isSro = true;
      }, 10);

  }
